Brain damage, which can be caused by conditions such as amnesia and Alzheimer's Disease, doesn't just erase memories. In a shocking new discovery, it turns out brain damage can also cause its sufferers to experience false memories of familiarity.

The perirhinal cortex is one of the brain's most important memory centers, charged with forming memories by arranging sensory information into a detailed, comprehensible picture of the event. It's also one of the most vulnerable sections of the brain to damage, as Cambridge psychologist Lisa Saksida explains: "The perirhinal cortex is one of the first regions that is affected in Alzheimer's disease, and it is very often damaged in cases of amnesia, so specific damage in this region is highly relevant to both of these conditions."

The loss of this cortex forces the brain to improvise when it comes to making new memories, relying on simpler parts of the brain to fill in and help make sense of incoming information. To do this, the brain has to start relying on far simpler features that can be found on a wide variety of objects. Saksida explains how this can lead to problems:

"The remaining representations of simpler features of objects are relatively easily confused, and as a result, false memories are generated."

Researchers first realized the possibility of false memory creation while running computer simulations of animal brain damage. The simulation predicted animals would then start to perceive objects they had never seen before as familiar. This was a counterintuitive result, and they decided to test it out using rats that had been surgically brain-damaged. Here's what the experiment involved:

The researchers first gave the rats three minutes to familiarize themselves with a specific "junk object" – a small, complex and colorful object similar to a small garden gnome or plastic toy train that you might find in a dollar store. Next, they held the rats in a separate cage environment for an hour before releasing the rats into an environment where they encountered either the old object or a new object.

Normal rats spent more time exploring the new objects, and less time with the old familiar object. But the brain-damaged rats spent less time exploring the new object – a sign that they treated the new object as if it were familiar. If the rats had simply forgotten about having encountered the old object, they would have spent more time exploring both the old and new objects.

Saksida and her colleagues suspected that the rats had built up false memories by seeing similar shapes in their holding area, and so they retested the rats by first placing them in a darkened space where they couldn't see anything. In this new environment, they couldn't build up any false memories, and they then performed the experiment completely normally.

The researchers have yet to repeat these experiments on humans, although they hope to do so in the future. As Saksida points out, there's already some evidence that similar processes occur in humans:

"There is already some evidence in the literature that people with various types of memory impairment (including Alzheimer's disease) perform better on memory tests if they spend time before the test in a dark, quiet environment, which is consistent with what we found here."

This does not, it should be stressed, explain everything about how brain damage works, particularly in the more complex minds of humans. However, Saksida is confident this will help put together a clearer picture of what happens to the brain when it suffers damage.